sos is a Scheme object system derived from Tiny clos 1, which in turn was loosely derived from clos, the Common Lisp Object System. Its basic design and philosophy is closely related to Tiny clos, but there are differences in naming and interface.
This document is a reference manual, and as such does not attempt to teach the reader about object-oriented programming. It is assumed that you already have a passing familiarity with clos and with Scheme.
In the procedure descriptions that follow, certain argument names imply restrictions on the corresponding argument. Here is a table of those names. The parenthesised name in each entry is the name of the predicate procedure that the argument must satisfy.
symbol?); sometimes this is also allowed to be
 Tiny clos was written by Gregor Kiczales of Xerox parc; sos is derived from version 1.2 of Tiny clos.